This invention relates, in general, to an improvement in noise suppression structures and more specifically, but not by way of limitation, to a method of adhesively bonding a perforated sheet to a microporous sheet for such structures.
Structures having a perforated plate with a microporous material bonded to one side have been found to have great utility in noise suppression structures used in gas turbine aircraft engine housings or the like. Difficulties have been encountered in bonding the two porous members together in a manner that is suitable for volume production and which does not unacceptably reduce porosity by covering at least some perforations or pores.
Spraying the perforated plate with a liquid adhesive, then pressing a microporous material thereagainst has not been entirely effective because of wicking of the liquid adhesive into the microporous material and closing some pores in edges which overlap the perforations. Also, handling the wet, tacky, sprayed plate and trying to spread the porous material thereover without wrinkling thereof is very difficult, especially in volume production circumstances. This technique seems to be effective only where two perforated plates are to be bonded together, as described, for example by Elder et al. in U.S. Pat. No. 3,770,560. Even there, the use of wet adhesives is cumbersome and slow.
Attempts have been made to impregnate the porous material with sufficient adhesive to permit effective bonding to another surface, but not so much as to close pores, as described by Hilliard et al. in U.S. Pat. No. 4,111,081. Consistent critical impregnation is difficult, with insufficient resin resulting in poor bonding and excess resin severely reducing porosity. Some pores will always be closed and/or reduced in size.
Dry adhesive layers of the sort described by Martin et al. in U.S. Pat. No. 2,951,004, which can be laid up between sheets to be laminated, and then heated to melt and become adhesive are now well known. While very effective in volume lamination of imperforate sheets, they have not been useful for bonding perforate or porous sheets, since the adhesive covers the perforations or pores. As described by Kazama in U.S. Pat. No. 4,150,186, the hot melt adhesive film can even be caused to deliberately fill holes by using a vacuum to draw part of the film into a hole to fill it and form a locking button.
Thus, there remains a need for an improved method of bonding a perforated sheet to microporous materials in a manner that is suitable for volume production and which does not block any perforations or pores.